Fuel assembly for a nuclear boiling water reactor

ABSTRACT

A fuel assembly for a nuclear power boiling water reactor, including: a fuel channel defining a central fuel channel axis, fuel rods, each having a central fuel rod axis, at least 3 water channels for non-boiling water, each water channel having a central water channel axis and each water channel having a larger cross-sectional area than the cross-sectional area of (the average) fuel rod. The fuel rods comprise a first group of full length fuel rods and a second group of shorter fuel rods. The fuel assembly comprises at least 5 fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these at least 5 fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.

FIELD

The present invention concerns a fuel assembly for a nuclear powerboiling water reactor.

BACKGROUND

In a fuel assembly for a nuclear boiling water reactor, there are anumber of fuel rods, which comprise a nuclear fuel material. When thefuel assembly is in operation in a nuclear reactor, a cooling medium,usually water, flows up through the fuel assembly. This water fulfilsseveral functions. It functions as a cooling medium for cooling the fuelrods such that they will not be overheated. The water also serves as aneutron moderator, i.e. the water slows down the neutrons to a lowerspeed. Thereby, the reactivity of the reactor is increased.

Since the water flows upwards through the fuel assembly, in the upperpart of the fuel assembly, the water has been heated to a larger extent.This has as a consequence that the portion of steam is larger in theupper part of the fuel assembly than in the lower part. Since steam hasa relatively low density, the steam in the upper part of the fuelassembly is a poorer moderator than the water in the lower part of thefuel assembly. Furthermore, it is the case that cold water is a bettermoderator than warm water. This means that the largest moderation isobtained when the reactor is out of operation, i.e. when it is cool. Thereactivity of a reactor depends on the amount of nuclear fuel materialand on the amount of moderator. The reactivity in a cool reactor isthereby higher than the reactivity in a warm reactor. To enable safeshutdown, there are requirements on a highest allowed reactivity whenthe reactor is out of operation. An aim is thus that the reactor has areactivity as high as possible when the reactor is in operation at thesame time as the reactivity may not be too high when the reactor is outof operation.

It should be mentioned that the water does not only have a moderatingfunction. The water functions in fact also as a neutron absorber. Inthis context, the expression over-moderation is often used. Thereby ismeant that the absorbing function of the water dominates over itsmoderating function. Such an over-moderation thus leads to a loweredreactivity. This means that the requirement on a highest allowedreactivity when the reactor is out of operation is more easily fulfilledif the amount of water leads to over-moderation.

Another requirement is that the cooling of the fuel rods is sufficientsuch that a so-called dry-out does not occur. Dry-out means that thewater film which exists on the surface of the fuel rods disappears or isbroken in limited areas. This leads to a locally deteriorated heattransfer between the fuel rod and the water flowing through the fuelassembly. This leads in its turn to an increased wall temperature of thefuel rods. The increased wall temperature may lead to serious damage onthe fuel rod.

It is desired to achieve a distribution of fission power over thecross-section of the fuel assembly which is more uniform such that theso-called radial peaking factor will be reduced. This means that theassembly can be operated to a higher total power before any individualfuel rod reaches its limits in terms of dry-out margin and other safetyrelated parameters.

In order to fulfil the different safety requirements, to obtain asufficient cooling of the fuel rods, and, at the same time, to obtain ahigh reactivity during operation, a large number of different technicalsolutions have been proposed.

Examples of different designs of fuel assemblies for a nuclear boilingwater reactor can be seen EP 1551034 A2, U.S. Pat. No. 5,068,082 andU.S. Pat. No. 4,968,479.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a fuel assembly for anuclear boiling water reactor with an improved cold shut-down margin,i.e. the reactivity should be sufficiently low when the nuclear reactoris shut down (cold condition). A further object is to provide such afuel assembly which has a high reactivity when the nuclear reactor is inoperation (hot condition). A further object is to provide such a fuelassembly which has a fission power distributed evenly over thecross-section of the fuel assembly. A further object is to provide sucha fuel assembly which has a reduced pressure drop in the upper two-phaseflow region, in order to improve thermal-hydraulic stability.

The above objects are achieved by a fuel assembly for a nuclear powerboiling water reactor, comprising:

a fuel channel (6) extending in and defining a length direction (L) ofthe fuel assembly (4) and defining a central fuel channel axis (8)extending in said length direction,

fuel rods (10) positioned such that they are surrounded by said fuelchannel (6), each fuel rod having a central fuel rod axis (12) extendingsubstantially in said length direction,

water channels (14) positioned such that they are surrounded by saidfuel channel, the water channels (14) being configured and positionedfor, during operation, allowing non-boiling water to flow through thewater channels, each water channel having a central water channel axis(16) extending substantially in said length direction,

wherein said fuel rods comprise a first group of fuel rods and a secondgroup of fuel rods,

wherein each fuel rod in said first group is a so-called full lengthfuel rod which extends from a lower part of the fuel assembly to anupper part of the fuel assembly,

wherein each fuel rod in said second group extends from said lower partof the fuel assembly and upwards, but does not reach as high up as saidfull length fuel rods,

wherein said water channels (14) of the fuel assembly (4) comprises atleast 3 water channels (14), each of which has a cross-sectional areawhich is at least twice as large as the cross-sectional area of each oneof said fuel rods (10), or, in case the fuel assembly has fuel rods ofdifferent cross-sectional areas, at least twice as large as the averagecross-sectional area of the fuel rods,

wherein said at least 3 water channels (14) are positioned such thatthere is no further water channel of the fuel assembly, the centralwater channel axis (16) of which is closer to the central fuel channelaxis (8) than the central water channel axis (16) of each of said atleast 3 water channels,

characterized in that the fuel assembly (4) comprises at least 5 fuelrods which belong to said second group and which are positioned suchthat the central fuel rod axis (12) of each of these at least 5 fuelrods is closer to the central fuel channel axis (8) than any of thewater channel axes (16) of the water channels (14) of the fuel assembly.

Since the fuel assembly comprises a relatively large number of shorterfuel rods in a central position of the fuel assembly, together with atleast three, relatively large, water channels, which are positioned“outside” of the central short fuel rods, a large volume of water iscreated in the central upper region of the fuel assembly, when thenuclear reactor is in the cold condition. This region is thenovermoderated and the cold reactivity is reduced. Therefore, an improvedshut-down margin is obtained. Furthermore, since at least three,relatively large, water channels are used, and since these waterchannels are “spread out” in the fuel assembly (since they arepositioned outside of the defined central shorter fuel rods), thesewater channels will be located near many of the fuel rods arranged inthe fuel assembly. Therefore a good moderation is obtained in the hotcondition, which means that the reactivity of the nuclear reactor willbe high. Moreover, because the reactivity is more evenly spread out tomore fuel rods, the distribution of fission power over the cross-sectionof the fuel assembly will be more uniform. Also because of therelatively large number of central shorter fuel rods, a larger volumewithout any fuel rods is created in the upper part of the fuel assembly.This means that the pressure drop will be reduced in the upper part ofthe fuel assembly as desired.

In an operating boiling water reactor the moderation changes up throughthe reactor due to the formation of steam and hence the reduced density.This gives a higher conversion in the upper part, i.e. more productionof Pu-239 from U-238, with higher reactivity at cold condition as aresult. This problem has in the prior art been solved by use of shorterfuel rods. Since the shorter rods have negative side effects, theirnumbers, lengths and positions are crucial.

The large open region above the central shorter fuel rods also enhancesnatural steam separation which reduces the average steam volume andhence increases moderation at hot conditions. A separation of steam andwater where the steam travels upwards through the assembly at a higherspeed reduces the average steam volume. This process requires largeropen areas than the empty positions above single shorter fuel rods.

The fuel assembly thus comprises at least 3 water channels with thecross-sectional area as defined in claims 1. The fuel assembly couldalso comprise one or more water channels with a smaller cross-sectionalarea. However, according to a preferred embodiment, the fuel assemblydoes not comprise any such water channel with a smaller cross-sectionalarea. Nevertheless, if the fuel assembly comprises one or more waterchannels with a smaller cross-sectional area, then, as defined in claim1, none of such possible water channels with a smaller cross-sectionalarea is positioned such that its central water channel axis is closer tothe central fuel channel axis than the central water channel axis ofeach of said at least 3 water channels with the defined largercross-sectional area.

The following may be noted concerning the expressions used in theclaims.

A fuel channel can also be called for example a box wall or channelwall.

The fuel channel is normally quite long (for example about 4 m) comparedto its width (for example about 1.5 dm). It therefore has a lengthdirection.

In use in a nuclear reactor, the fuel assembly, and the fuel channel,preferably extend mainly in the vertical direction. The length directionis thus, in use, the vertical direction. The lower and upper parts ofthe fuel assembly therefore refer to the fuel assembly as seen in theintended use position.

The fuel rods could be slightly tilted. Hence, it is specified that thefuel rod axis extends substantially in the length direction. However,preferably the fuel rods are not tilted and therefore the fuel rod axisextends only in the length direction.

Preferably the fuel rods are straight. However, the fuel rods may alsobe somewhat bent. The defined central fuel rod axis would in that casefollow the bent shape of the fuel rod, i.e. also the central fuel rodaxis would in that case be bent.

Similarly for the water channels. The water channels could be slightlytilted. Hence it is specified that the water channel axis extendssubstantially in the length direction. However, preferably the waterchannels are not tilted and therefore the water channel axis extendsonly in the length direction. Also, preferably the water channels arestraight. However, the water channels could also be bent. The definedcentral water channel axis would in that case follow the bent shape ofthe water channel, i.e. also the central water channel axis would inthat case be bent.

A water channel in this application thus means an enclosure (for exampleof a tubular shape) which is positioned in the fuel assembly and whichis arranged for allowing non-boiling water to flow therethrough.

Furthermore, preferably the water channel has a constant cross-sectionalarea over at least 80% of its length, preferably over its whole length(the cross-sectional area could change somewhat close to the end(s) ofthe water channel). However, according to an alternative embodiment, thecross-sectional area of the water channel may vary along its length. Forexample, the cross-sectional area may become larger at a level above thementioned at least 5 shorter central fuel rods.

When the cross-sectional area of the water channels and the fuel rodsare compared with each other, this comparison concerns the same level inthe fuel assembly (in case the water channels or, possibly, the fuelrods would have a varying cross-sectional area). In particular, thecomparison applies to the lower part of fuel assembly, where the shorterfuel rods are positioned.

The cross-sectional area relates to the area defined by the outerperiphery of the water channels or the fuel rods.

The nuclear reactor is preferably a light water reactor.

According to one embodiment of a fuel assembly according to theinvention, there is no full length fuel rod, the central fuel rod axisof which is positioned closer to the central fuel channel axis than thecentral fuel rod axis of any of said at least 5 fuel rods. This factensures that there will be a relatively large region above the mentionedcentrally located shorter fuel rods. Consequently, a space is providedabove these shorter rods for a relatively large volume of water, whichwill improve the shut-down margin.

According to another embodiment of a fuel assembly according to theinvention, the fuel assembly comprises at least 7 fuel rods which belongto said second group and which are positioned such that the central fuelrod axis of each of these at least 7 fuel rods is closer to the centralfuel channel axis than any of the water channel axes of the waterchannels. With at least 7 such fuel rods, an even larger central spaceis created, which means a further improved shut-down margin.

According to an embodiment, there are also no more than 8 fuel rodswhich belong to said second group and which are positioned such that thecentral fuel rod axis of each of these fuel rods is closer to thecentral fuel channel axis than any of the water channel axes of thewater channels. According to a preferred embodiment there are exactly 8such fuel rods which belong to said second group and which arepositioned such that the central fuel rod axis of each of these fuelrods is closer to the central fuel channel axis than any of the waterchannel axes of the water channels. By having 8 such fuel rods, anoptimized design is achieved.

According to another embodiment of a fuel assembly according to theinvention, there is no full length fuel rod, the central fuel rod axisof which is positioned closer to the central fuel channel axis than thecentral fuel rod axis of any of said at least 7 fuel rods. Similarly toabove, it is thereby ensured that there will be a large space for waterabove the short central fuel rods.

According to another embodiment of a fuel assembly according to theinvention, each of said at least 5 fuel rods or each of said at least 7fuel rods has a length that is less than 0.50 times the length of saidfull length fuel rods. Since the fuel rods are that short, it is ensuredthat there is a large space for water above the fuel rods.

According to a preferred embodiment, each of said at least 5 fuel rodsor each of said at least 7 fuel rods has a length that is between 0.29and 0.45 times the length of said full length fuel rods. With such shortfuel rods an even larger volume for water is created.

According to another embodiment, there is no fuel rod which is such thatit is longer than 0.50 times the length of said full length fuel rodsand has a central fuel rod axis which is positioned closer to thecentral fuel channel axis than the central fuel rod axis of any of saidat least 5 fuel rods or said at least 7 fuel rods. Similarly to theabove explanation, by ensuring that there are no longer fuel rods amongthe mentioned central shorter fuel rods, a large, undisturbed, space forwater is created.

According to another embodiment, the fuel assembly comprises no morethan 3 of said at least 3 water channels. It has been found that the useof three such, relatively large, water channels is optimal for achievinggood moderation in the hot condition, at the same time as there is stillsufficient space in the fuel assembly for a relatively large number offuel rods.

According to a preferred embodiment, the fuel assembly does not compriseany other water channels either (i.e. also no water channel with across-sectional area which is less than twice as large as thecross-sectional area of each one of said fuel rods, or, in case the fuelassembly has fuel rods of different cross-sectional areas, less thantwice as large as the average cross-sectional area of the fuel rods).

According to another embodiment, each one of said at least 3 waterchannels has a cross-sectional area which is between 3.0 and 10.0,preferably between 4.0 and 8.0, times the cross-sectional area of eachone of said fuel rods, or, in case the fuel assembly has fuel rods ofdifferent cross-sectional areas, between 3.0 and 10.0, preferablybetween 4.0 and 8.0, times the average cross-sectional area of the fuelrods. With such relatively large water channels, a sufficiently highamount of non-boiling water will flow through the fuel assembly. Thisensures a good moderation, i.e. a high reactivity.

According to another embodiment, each of said at least 3 water channelshas a circular cross-section, at least in the portion of the waterchannel that is located at the level of said at least 5 fuel rods orsaid at least 7 fuel rods. From a flow dynamic point of view, it isadvantageous to use round water channels. Furthermore, it is easy tomanufacture and position such round water channels in the fuel assembly.

According to another embodiment, the fuel assembly comprises no morethan 19 fuel rods, preferably no more than 16 fuel rods, more preferredno more than 14 fuel rods, each of which fulfils the followingcriterion: the distance between the central fuel rod axis and thecentral fuel channel axis is less than the distance between the centralwater channel axis of at least one of said at least 3 water channels andthe central fuel channel axis. It is thereby ensured that the waterchannels are not positioned too far towards the periphery of the fuelassembly. This means that a good moderation, and a high reactivity, formany fuel rods is achieved, and consequently also an evenly distributedfission power.

According to preferred embodiments, the fuel assembly comprises 7-21,preferably 10-18, more preferred 13-15, most preferred 14 fuel rodswhich fulfil the mentioned criterion. This has appeared to ensure anoptimal positioning of the water channels.

This means that the water channels are positioned near the central shortfuel rods (said at least 5 or at least 7 fuel rods). Preferably, each ofthe water channels is positioned next to at least two of the centralshort fuel rods, such that there is no further fuel rod positionedbetween the respective water channel and said at least two central shortfuel rods.

According to another embodiment, the fuel assembly comprises asubstantially regular pattern of fuel rod positions, wherein each one ofsaid at least 3 water channels is positioned such that it replaces 4fuel rods in this substantially regular pattern. Such a design is quiteeasy to implement in a fuel assembly.

The concept “substantially regular pattern” is used, since some fuelrods may be slightly displaced from the absolutely regular pattern.Preferably the regular pattern is in the form of rows and columns (whena cross-section of the fuel assembly is viewed).

According to another embodiment, the fuel assembly comprises 65-160,preferably 100-120, more preferred 105-113, most preferred 109 fuelrods. Such a relatively high number of fuel rods ensures that the fuelassembly can achieve an efficient heat transfer to the coolant, andbecause of the arrangement of the fuel rods and the water channels, agood moderation is obtained.

According to another embodiment, the fuel assembly comprises 2-8,preferably 4-6 fuel rods, each of which has a length of between 0.59 and0.79 times the length of said full length fuel rods. The arrangement ofsuch fuel rods contributes to the shut-down margin and to a reduction ofthe pressure drop in the upper part of the fuel assembly.

According to one embodiment, the fuel assembly comprises 8-16,preferably 11-13 fuel rods, each of which has a length that is between0.29 and 0.45 times the length of said full length fuel rods. With thisnumber of such short fuel rods, the shut-down margin is improved.

According to another embodiment, the fuel assembly comprises at least70, preferably at least 80, or at least 90 full length fuel rods. Anefficient heat transfer is obtained by using many full length fuel rods.

According to one embodiment, the fuel assembly comprises 5-20,preferably 10-15 fuel rods, each of which has a length of between 0.80and 0.95 times the length of said full length fuel rods. The arrangementof such fuel rods will reduce the pressure drop in the upper part of thefuel assembly, near the outlet for the water/steam.

According to another embodiment, the fuel assembly comprises:

a lower tie plate, positioned below the fuel rods, wherein a lower endof each of said at least 3 water channels is attached to said tie plate,

an upper lifting device, positioned above the fuel rods, including ahandle for gripping and lifting a bundle of fuel rods,

a plurality of spacer grids for holding the fuel rods, at least most ofthe spacer grids being attached to said at least 3 water channels,

attachment rods, attached at a lower end to the upper part of said atleast 3 water channels and at an upper end attached to said upperlifting device.

Such a design will make it easier to handle the bundle of fuel rods.Since the upper handle and lifting device is attached to the attachmentrods, which are attached to the water channels, which are attached tothe lower tie plate, and since the spacer grids hold the fuel rods andsince at least most of the spacer grids are attached to the waterchannels, it is possible to lift the whole bundle of fuel rods bygripping and lifting the handle.

According to one design principle, the fuel channel is permanently fixedto a bottom transition piece, which includes a debris filter, and thewhole fuel bundle as described above (including upper handle and liftingdevice, attachments rods, water channels, lower tie plate, and spacergrids) is lowered into the fuel channel and is resting freely on top ofthe transition piece.

According to an alternative design principle, the whole fuel bundle asdescribed above (including upper handle and lifting device, attachmentsrods, water channels, lower tie plate, and spacer grids) is permanentlyfixed to the transition piece, which includes a debris filter, and thefuel channel is placed over the fuel bundle and resting on the handle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a side view of a fuel assembly according toan embodiment of the invention.

FIG. 2 shows schematically a cross-section of an embodiment of a fuelassembly according to the invention.

DETAILED DESCRIPTION

An embodiment of the invention will now be described with reference toFIG. 1 and FIG. 2.

FIG. 1 shows schematically a side view of a fuel assembly 4 according toan embodiment of the invention. The fuel assembly 4 comprises a numberof fuel rods 10 and water channels 14. A lower tie plate 20 is arrangedbelow the fuel rods 10. A lower end of the water channels 14 is attachedto the tie plate 20. Above the fuel rods 10 an upper lifting device 22is arranged. The upper lifting device 22 has a handle 24 for grippingand lifting a bundle of fuel rods 10.

The fuel rods 10 are held by a plurality of spacer grids 26. It shouldbe noted that FIG. 1 schematically shows only an upper and lower part ofthe fuel assembly 4. According to an embodiment, the fuel assembly 4comprises ten spacer grids 26. The fuel assembly 4 also comprisesattachment rods 28, which at a lower end are attached to the upper partof the water channels 14 and which at an upper end are attached to theupper lifting device 22. All spacer grids 26, with one exception, areattached to the water channels 14. The upper spacer grid 26 ispositioned at the level of the attachment rods 28. The whole bundle offuel rods 10 is thus held together with the help of the water channels14, lower tie plate 20, attachment rods 28, upper lifting device 22 andspacer grids 26. It is therefore possible to lift the whole bundle offuel rods 10 by gripping and lifting at the handle 24.

With reference also to FIG. 2 the fuel assembly 4 will now be describedin more detail. The fuel assembly 4 comprises a fuel channel 6 whichsurrounds the bundle of fuel rods 10. In FIG. 1 the fuel channel 6 hasbeen removed in the viewing direction in order to make it possible tosee the components arranged inside the fuel channel 6. The fuel channel6 extends in a length direction L. The length direction L is normally,when the fuel assembly 4 is in use in a nuclear reactor, the verticaldirection. The fuel channel 6 has a central fuel channel axis 8 in saidlength direction L.

In FIG. 2, all the small circles refer to fuel rods 10. Each fuel rod 10has a central fuel rod axis 12 (shown only for one fuel rod 10), whichextends substantially in the length direction L.

The larger circles in FIG. 2 show the water channels 14. The waterchannels 14 are configured and positioned for allowing non-boiling waterto flow through the water channels 14, when the fuel assembly 4 is inuse in a nuclear reactor. Each water channel 14 has a central waterchannel axis 16 (shown only for one water channel 14 in FIG. 2), whichextends substantially in the length direction L.

The fuel assembly comprises a first group of full length fuel rods 10.The full length fuel rods are not marked in FIG. 2 (i.e. they are shownby empty circles). The full length fuel rods 10 extend from a lower partof the fuel assembly 4 to an upper part of the fuel assembly 4,preferably through all the spacer grids 26. It can be noted that in FIG.1 only full length fuel rods 10 are shown.

The fuel assembly 4 also comprises a second group of fuel rods 10. Thesecond group of fuel rods 10 extend from the lower part of the fuelassembly (like the full length fuel rods) but do not reach as high up asthe full length fuel rods.

The fuel rods 10 in said second group can have different lengths. In theshown embodiment, some fuel rods 10 are marked with one stroke. Thesefuel rods have a length of about 9/10 of the length of the full lengthfuel rods. In the shown embodiment, there are ten such fuel rods. Whenplacing these 9/10 fuel rods the most reactive positions next tonon-boiling water inside the water channels and outside the fuel channelare avoided. This is to minimize the negative impacts of having 1/10less uranium in these rods, while serving their purpose of reducingpressure drop near the assembly outlet.

The fuel rods 10 marked with two strokes (a cross) have a length ofabout ⅔ of the length of the full length fuel rods. In the shownembodiment there are four such fuel rods. These ⅔ fuel rods arepositioned halfway between the corner rods in the outer rows and columnsof the 11×11 fuel rod array. This is to reduce cold reactivity in theupper part of the fuel bundle which improves the shutdown margin late inthe fuel cycle when the power distribution has moved towards the top.

The fuel rods 10 marked with three strokes (a star) have a length ofabout ⅓ of the length of the full length fuel rods. In the shownembodiment there are twelve such fuel rods.

As shown in FIG. 2, the fuel assembly 4 according to this embodiment hasthree water channels 14. Each water channel 14 has a cross-sectionalarea which is about 5.5 times the cross-sectional area of each one ofthe fuel rods 10 (or, in case the fuel assembly 4 would have fuel rods10 of different cross-sectional areas, about 5.5 times the averagecross-sectional area of the fuel rods 10).

In the shown embodiment, there are only three water channels 14, i.e. nofurther water channels.

As shown in FIG. 2, there are eight centrally located fuel rods 10 ofthe shortest kind. These central short fuel rods 10 are positioned suchthat for each of these fuel rods 10 it is the case that the distancebetween the fuel rod axis 12 and the central fuel channel axis 8 isshorter than the distance between any of the water channel axes 16 ofthe water channels 14 and the fuel channel axis 8.

It should be noted that FIG. 2 shows a schematic cross-section of thefuel assembly 4 in the lower part of the fuel assembly (where also allthe shorter fuel rods 10 are present).

There is no longer fuel rod 10 (no ⅔ fuel rod or 9/10 fuel rod or fulllength fuel rod) which is positioned closer to the central fuel channelaxis 8 than the central fuel rod axis 12 of any of the eight centralshort fuel rods 10. Above the eight short central fuel rods 10, there isthus an empty space for water in the fuel assembly 4.

In addition to the eight central short ⅓ fuel rods, there are a furtherfour such short fuel rods 10 located in the corners of the fuel assembly4.

Each of the water channels 14 has a circular cross-section, at least inthe lower part of the fuel assembly 4 where the shorter central fuelrods 10 are arranged.

In addition to the mentioned eight central short fuel rods 10, the fuelassembly 4 comprises a further six fuel rods, each of which fulfils thefollowing criterion. The distance between the central fuel rod axis 12and the central fuel channel axis 8 is less than the distance betweenthe central water channel axis 16 of at least one of the three waterchannels 14 and the central fuel channel axis 8. In the shownembodiment, there are 14 fuel rods 10 that fulfil the mentionedcriterion. These fuel rods 10 are located inside the dashed lines inFIG. 2. Each water channel 14 is positioned next to at least some of theeight centrally located short fuel rods 10.

As can be seen in FIG. 2, the fuel assembly 4 comprises a substantiallyregular pattern of fuel rod positions. Each one of the water channels 14is positioned such that it replaces four fuel rods 10 in this regularpattern.

In the shown embodiment, the fuel assembly 4 thus comprises 83 fulllength fuel rods 10, ten 9/10 length fuel rods, four ⅔ length fuel rodsand twelve ⅓ length fuel rods.

The shown embodiment provides an advantageous fuel assembly 4 with whichthe above described objects and advantages of the invention areachieved.

The present invention is not limited to the examples described herein,but can be varied and modified within the scope of the following claims.

1-15. (canceled)
 16. A fuel assembly for a nuclear power boiling waterreactor, comprising: a fuel channel extending in and defining a lengthdirection of the fuel assembly and defining a central fuel channel axisextending in said length direction; fuel rods positioned such that theyare surrounded by said fuel channel, each fuel rod having a central fuelrod axis extending substantially in said length direction; and waterchannels positioned such that they are surrounded by said fuel channel,the water channels being configured and positioned for, duringoperation, allowing non-boiling water to flow through the waterchannels, each water channel having a central water channel axisextending substantially in said length direction, wherein said fuel rodscomprise a first group of fuel rods and a second group of fuel rods,wherein each fuel rod in said first group is a so-called full lengthfuel rod which extends from a lower part of the fuel assembly to anupper part of the fuel assembly, wherein each fuel rod in said secondgroup extends from said lower part of the fuel assembly and upwards, butdoes not reach as high up as said full length fuel rods, wherein saidwater channels of the fuel assembly comprise at least three and no morethan three water channels, each of which has a cross-sectional areawhich is at least twice as large as a cross-sectional area of each oneof said fuel rods, or, in case the fuel assembly having fuel rods ofdifferent cross-sectional areas, at least twice as large as the averagecross-sectional area of the fuel rods, wherein said at least three andno more than three water channels are positioned such that there is nofurther water channel of the fuel assembly, the central water channelaxis of which is closer to the central fuel channel axis than thecentral water channel axis of each of said at least three and no morethan three water channels, and wherein the fuel assembly furthercomprises at least five fuel rods which belong to said second group andwhich are positioned such that the central fuel rod axis of each ofthese at least five fuel rods is closer to the central fuel channel axisthan any of the water channel axes of the water channels of the fuelassembly.
 17. A fuel assembly according to claim 16, wherein there is nofull length fuel rod, the central fuel rod axis of which is positionedcloser to the central fuel channel axis than the central fuel rod axisof any of said at least five fuel rods.
 18. A fuel assembly according toclaim 16, comprising at least seven fuel rods which belong to saidsecond group and which are positioned such that the central fuel rodaxis of each of these at least seven fuel rods is closer to the centralfuel channel axis than any of the water channel axes of the waterchannels.
 19. A fuel assembly according to claim 18, wherein there is nofull length fuel rod, the central fuel rod axis of which is positionedcloser to the central fuel channel axis than the central fuel rod axisof any of said at least seven fuel rods.
 20. A fuel assembly accordingto claim 16, wherein each of said at least five fuel rods has a lengththat is less than 0.50 times the length of said full length fuel rods.21. A fuel assembly according to claim 18, wherein each of said at leastseven fuel rods has a length that is less than 0.50 times the length ofsaid full length fuel rods.
 22. A fuel assembly according to claim 20,wherein there is no fuel rod which is such that it is longer than 0.50times the length of said full length fuel rods and has a central fuelrod axis which is positioned closer to the central fuel channel axisthan the central fuel rod axis of any of said at least five fuel rods.23. A fuel assembly according to claim 21, wherein there is no fuel rodwhich is such that it is longer than 0.50 times the length of said fulllength fuel rods and has a central fuel rod axis which is positionedcloser to the central fuel channel axis than the central fuel rod axisof any of said at least seven fuel rods
 24. A fuel assembly according toclaim 16, wherein the cross-sectional area of each one of said at leastthree and no more than three water channels is between 3.0 and 10.0times the cross-sectional area of each one of said fuel rods.
 25. A fuelassembly according to claim 24, wherein the cross-sectional area of eachone of said at least three and no more than three water channels isbetween 4.0 and 8.0 times the cross-sectional area of each one of saidfuel rods.
 26. A fuel assembly according to claim 16 wherein the fuelassembly has fuel rods of different cross-sectional areas and whereinthe cross-sectional area of each one of said at least three and no morethan three water channels is between 3.0 and 10.0 times the averagecross-sectional area of the fuel rods.
 27. A fuel assembly according toclaim 26 wherein the fuel assembly has fuel rods of differentcross-sectional areas and wherein the cross-sectional area of each oneof said at least three and no more than three water channels is between4.0 and 8.0 times the average cross-sectional area of the fuel rods. 28.A fuel assembly according to claim 16, wherein each of said at leastthree and no more than three water channels has a circularcross-section, at least in the portion of the water channel that islocated at the level of said at least five fuel rods or said at leastseven fuel rods.
 29. A fuel assembly according to claim 16, wherein thefuel assembly comprises no more than 19 fuel rods, each of which fulfilsthe following criterion: the distance between the central fuel rod axisand the central fuel channel axis is less than the distance between thecentral water channel axis of at least one of said at least three and nomore than three water channels and the central fuel channel axis.
 30. Afuel assembly according to claim 29, wherein the fuel assembly comprisesno more than 16 fuel rods, each of which fulfils the followingcriterion: the distance between the central fuel rod axis and thecentral fuel channel axis is less than the distance between the centralwater channel axis of at least one of said at least three and no morethan three water channels and the central fuel channel axis.
 31. A fuelassembly according to claim 30, wherein the fuel assembly comprises nomore than 14 fuel rods, each of which fulfils the following criterion:the distance between the central fuel rod axis and the central fuelchannel axis is less than the distance between the central water channelaxis of at least one of said at least three and no more than three waterchannels and the central fuel channel axis.
 32. A fuel assemblyaccording to claim 16, wherein the fuel assembly comprises asubstantially regular pattern of fuel rod positions, wherein each one ofsaid at least three and no more than three water channels is positionedsuch that it replaces four fuel rods in this substantially regularpattern.
 33. A fuel assembly according to claim 16, wherein the fuelassembly comprises 65-160 fuel rods.
 34. A fuel assembly according toclaim 33, wherein the fuel assembly comprises 100-120 fuel rods.
 35. Afuel assembly according to claim 34, wherein the fuel assembly comprises105-113 fuel rods.
 36. A fuel assembly according to claim 35, whereinthe fuel assembly comprises 109 fuel rods.
 37. A fuel assembly accordingto claim 16, wherein the fuel assembly comprises 2-8 fuel rods, each ofwhich has a length of between 0.59 and 0.79 times the length of saidfull length fuel rods.
 38. A fuel assembly according to claim 37,wherein the fuel assembly comprises 4-6 fuel rods, each of which has alength of between 0.59 and 0.79 times the length of said full lengthfuel rods.
 39. A fuel assembly according to claim 16, wherein the fuelassembly comprises at least 70 full length fuel rods, each of which hasa length of between 0.59 and 0.79 times the length of said full lengthfuel rods.
 40. A fuel assembly according to claim 39, wherein the fuelassembly comprises at least 80 full length fuel rods.
 41. A fuelassembly according to claim 40, wherein the fuel assembly comprises atleast 90 full length fuel rods.
 42. A fuel assembly according to claim16, further comprising: a lower tie plate, positioned below the fuelrods, wherein a lower end of each of said at least three and no morethan three water channels is attached to said tie plate; an upperlifting device, positioned above the fuel rods, including a handle forgripping and lifting a bundle of fuel rods; a plurality of spacer gridsfor holding the fuel rods, at least most of the spacer grids beingattached to said at least three and no more than three water channels;and attachment rods, attached at a lower end to the upper part of saidat least three and no more than three water channels and at an upper endattached to said upper lifting device.